HART (Highway Addressable Remote Transducer) is a global protocol standard for intelligent process instrumentation. Over 30 million HART-compatible devices are currently installed in systems throughout the world. Its full potential is only used by 10% of devices, however.
The WirelessHART standard was developed by the HART Foundation to enable users to more effectively utilize the capabilities of their HART-enabled devices. The Phoenix Contact WirelessHART products are designed to allow rapid integration into existing applications.
In 2007, the HART Communication Foundation released the first wireless standard specifically developed for industrial applications. WirelessHART is a subset of the HART 7 standard and is an energy-efficient wireless technology designed for process measurement applications, eliminating or augmenting 4 - 20 mA signal wires.
The WirelessHART standard is based on a robust and reliable technology. This can be used easily with just a few configuration parameters and with virtually all device types.
Wireless transmission with WirelessHART in the form of IEEE 802.15.4 wireless modules with a 10 mW wireless transceiver can be used throughout the world in the license-free 2.4 GHz ISM frequency band. Wireless modules according to 802.15.4 and ZigBee are often used as the basis for energy-efficient wireless networks.
The WirelessHART protocol uses the top four layers of the OSI model and offers considerable advantages over other wireless systems with a low power consumption. It enables reliable parallel operation and offers numerous possibilities for the optimum management of wireless channels.
This includes a combination of the DSSS and FHSS frequency spread and frequency hopping methods, CCA (Clear Channel Assessment) technology, short transmission times, and blacklisting functionality.
The main feature of WirelessHART is its ability to construct and configure meshed networks itself. This enables every end device in the network to communicate redundantly via several different paths. Here the open air range is approx. 250 meters and the physical size of the installation is generally 50 to 100 meters.
This guarantees that fully functional and ready to use network nodes are available at all times. Full mesh networks organize themselves fully automatically. They are also self-healing, which guarantees long-term reliability and predictability, even under changing environmental conditions.
Devices in meshed networks can automatically detect other devices, measure the strength of the wireless signal, and acquire synchronization and frequency hopping data. They can also establish or interrupt connections to other devices according to requirements. Networks and network nodes have a unique ID that enables several networks to be operated in parallel without unintentionally sharing data or misrouting signals.
Error-free wireless channel management and fully functional network construction rely on the precise timing of data transmission.
WirelessHART networks employ Time Division Multiple Access (TDMA) to manage how the spectrum is used over time. Every transmission takes place in a 10 ms window (time slot) and on one of the 15 channels with very precise timing. If no communication is taking place, devices go into a standby mode to conserve energy (most devices are battery-powered).
Information security is guaranteed by encryption, authentication, and data integrity functions. The 128-bit AES encryption protects data against unauthorized access by third parties.
During the authentication step, the validity of the sender identity is checked based on packet source addresses, which are protected by so-called 32-bit Message Integrity Codes (MICs). The integrity guarantee also ensures that information is transmitted unchanged by the same MIC. Additionally, frequency hopping provides some level of data security because of the pseudo-random hop sequence.
To ensure that a secure network is formed, all devices must be authenticated before they join the network. The network devices regularly send and publish network information in the form of small data packets, so-called beacons.
In order to join the network, a new device must first receive one of these data packets. Once a beacon has been received, the new device sends a join request to the higher-level WirelessHART network manager.
The join request consists of the device’s programmed network ID, join key, and other identifying information. If the network ID and join key are accepted by the network manager, it will respond to the new device with authentication information, the communication path in the network, and the time slots during which the device should transmit data.
After this, the device is completely joined to the network and can begin transferring and publishing data.
|WirelessHART gateway||WirelessHart measuring device||WirelessHART adapter|
|Scope of functions||Wireless access point|
of existing devices
|Field of application||Connection to the host system||Wireless measurement||Connection of wired|
measuring devices to WirelessHART
According to the HART standard, the so-called WirelessHART gateway is a device that consists of three components:
WirelessHART devices (usually wireless measuring devices) consist of a wireless module that is combined with measuring or monitoring functions. They enable existing systems to be extended easily and can be used immediately in new systems.
Measurement points previously inaccessible due to cabling costs or environmental restrictions can now be integrated with ease. WirelessHART devices can be powered by solar, line, loop or battery.
The WirelessHART adapter is used to connect existing wired HART devices to a WirelessHART network. The adapter connects to the existing 4 - 20 mA wiring to gather the HART signal while it remains intact and functional.
It is possible for one WirelessHART adapter to collect HART signals from multiple devices, resulting in lower installation costs. WirelessHART adapters can be powered by loop, line or battery. The primary application for adapters is to gather HART signals from previously installed HART devices that are connected to a host with no HART capability.